If there is something fishy with mail in ballots I think they’ll find it. Although it sounds like a conspiracy theory, i think Trump campaign would not pursue the issue if they weren’t quite confident about it.
As far as states announcing early winners and the effect it has on subsequent voting, that seems like a plausible suppositions, but unless it’s been shown scientifically, it just stays a supposition. I myself was wondering why VA and RI for that matter, were called early cause exit polls would have given it to trump cause he was leading both mid way through the count, but I guess those a clear pro-dem states and the news media got it right in the end anyway.